Gas-electric hybrids, propelled by electric motors at low speeds, are well-known for their quiet ride and great mileage. But their silence isn’t always golden.

Some researchers and safety groups say that quiet operation — “hybrid creep” — can pose risks for unsuspecting pedestrians and the blind, who use sound cues.

Advocates for the blind have sought the addition of artificial noises in hybrids for several years, concerned that the expected sales growth of hybrids could lead to more pedestrian fatalities and injuries. Hybrids account for about 2 percent of new car sales each year but auto companies are expected to boost production in advance of tougher fuel efficiency standards this decade.

“This is an example of too much of a good thing,” said John Pare, executive director for strategic initiatives with the National Federation of the Blind. “Cars got quieter, that was good. Suddenly, they got to be so quiet that it added an element of danger.”

The government’s auto safety agency said in a research report last year that hybrid vehicles are twice as likely to be involved in pedestrian crashes at low speeds compared with cars with conventional engines. The study by the National Highway Traffic Safety Administration examined circumstances in which the vehicles were slowing down or coming to a stop, backing up or entering or departing a parking space.

More than 4,300 pedestrians were killed in 2008, according to the most recent data available. The government has been researching the safety risks that hybrids and electrics could pose for pedestrians, particularly the blind, along with the elderly and children, for vehicles traveling at 20 mph or less.

Some green car advocates have questioned the need for the extra tones and noted that the requirement could add more noise to neighborhoods. Paul Scott, vice president of Plug In America, said the sounds could help under certain circumstances, but drivers should have the right to activate the tones.

Les Blomberg, who is the founder of the Noise Pollution Clearinghouse, said reducing noise from the loudest vehicles, such as trucks, buses and motorcycles, would increase the ability of pedestrians to detect sound. Adding sounds to hybrids, however, would simply enhance noise pollution and make it more difficult to hear an individual vehicle in traffic.